The Sharp is a subspecies of Gloople. When it senses danger, it will generate sharp nails on its skin, which will harden. In this state, a Sharp can easily impale humans, and its hardened skin provide it some protection, to a bullet-proof level. However, this has its disadvantages: an hardened Sharp can be knocked against other Glooples, effectively killing them.
A Sharp has this ability as a mecanism of hunting and defensive uses, but how a Sharp is able to do it? Sharps are Composed Of Mineral Gathering cell Points and specialized musculature (observations in Sharp's structure), as when a Sharp Is in this Shape, has a brighter look, that sugeres presence of hard cristals, in plates and protrusions.